After the end of the credits at the end of the game, you will be asked to create a save for a new game. All weapons except the 5th of each type will transfer over, however Peace Bell, Lucky Cat, Wood Mat, and Ink Pot are the only Holy Artifacts that transfer over. Brush techniques do not transfer over, but stats will transfer over.

Unlock the Okami Jukebox

Complete the game once to unlock the Okami Jukebox.

Loading Screens Trick

You can attain demon fangs to add to your inventory in the game by manipulating the loading screens. There are two in the game, one with a trail of paw prints followed by a picture of Issun, and by pressing X right before they appear. During long loading sessions, by carefully timing all these button presses all the way through, you will attain additional demon fangs which will be added to your inventory in the game.

More

When you finish the game, you have to complete the game a certain way in order to open most of these codes.